(1) Field of the Invention
The present invention relates to key telephones and more particularly to a microcomputer controlled key telephone line circuit for use in a key telephone system.
(2) Description of the Prior Art
Key telephone common circuits have traditionally been electromechanical in nature however recently electronic versions of these circuits have been patented. U.S. Pat. No. 3,952,169 issued on Apr. 20, 1976 to O. W. Vincent discloses the use of discrete logic and solid state switches to perform the key telephone functions. U.S. Pat. No. 4,057,693 issued on Nov. 8, 1977 to R. J. Angner however is more representative of the recent prior art. Angner uses a custom large scale integration chip to replace the logic which performs the key telephone common functions. However this chip only controls the functions for one line and is not suitable for use in a self-contained key telephone system such as the applicants' key telephone system. Bell Telephone Laboratories has recently developed a Com-Key 416 key telephone system which is similar to the applicants' key telephone system. However, in the Bell Com-Key system the common functions are provided by multiple custom large scale integration chips designed specifically to perform their limited purpose. Also these chips provide only logic control and interrrupt functions, while tone ringers and clock circuitry are external to the large scale integration chips. Also many of the required timing functions are performed by resistor-capacitor networks. Thus the Com-Key 416 key telephone system uses a large number of balky components to perform the key telephone line circuit functions.
Accordingly, it is an object of the present invention to provide a novel, low cost, minimum component technique of performing the key telephone line circuit functions.